(a) Field of the Invention
The present invention is mainly concerned with a process of extrusion molding a molding material comprising rubber, synthetic resin or the like which undergoes chemical changes caused by vulcanization or inter-molecular cross-linkage when molded and an apparatus therefore.
In this connection, it is to be noted that the present invention will be explained with reference to the instance of rubber unless specially mentioned hereinafter.
(b) Description of the Prior Art
Extrusion molding of molding materials such as rubbers or thermal cross-linkable synthetic resins has hitherto been carried out commonly by employing a method comprising pressure feeding a plasticized material by means of a screw or a different type extruder, extruding goods molded in a desired shape through a die head attached to the tip of the extruder, and subjecting said molded goods to further heating and vulcanizing treatment in another place by means of a continuous vulcanizing apparatus or the like, thereby obtaining products. For the purpose of the aforesaid vulcanization after molding, it has been common to employ processes of hot-chamber type, drum type, long-tube type, liquid medium vulcanization type, high frequency vulcanization type and others. However, all these processes are defective in that they are comparatively large-scaled, consume high energies and additionally are expensive. Moreover, these processes call for wide areas for equipment and further involve no small numbers of difficult points to be solved in processing respectively.
Properly speaking, the vulcanizing phenomena of rubbers are promoted by thermal hysteresis wherein temperature and time are employed as parameters. The vulcanization velocity of rubbers at the time of molding vulcanization is sensitive to temperatures, for instance in case the temperature rises by 10.degree. C. the time required for vulcanization is reduced to about 1/2 of that before the temperature rises. And, rubbers, once scorched, lose their previous plasticizability as well as fluidity to a non-reversible degree and become unable to flow. Moreover, rubbers are bad conductors to heat and should rather be said nearer to insulators. Although the values to be mentioned hereunder are variable in accordance with the percentages of components contained in the compounding rubber material, it is said that the heat conductivity of rubber is, for instance, 1/500 of that of iron, 1/5 of that of water and 6 times that of air. In this context, the specific heat of rubber is more than 4 times that of iron.
For the purpose of heating (inclusive of cooling) rubbers having properties as mentioned above by means of conventional extrusion molding machines, it has been common to employ a method comprising heating a molding material within a molding machine until it is fed to a die under an elevated pressure after it has passed through the respective sections of mixing by compression, softening by heating, transferring, etc., through a continuous structure comprising above mentioned sections. In other words, it is intended by the conventional heating method that a molding material temperature may be maintained, immediately before the die, at a predetermined temperature, for instance an optional fixed temperature in the range of 80.degree.-130.degree. C., by heating the molding material from the outer surface of the said structure wall with a heater placed thereon, said structure being made mainly of a thick steel material, or by heating (or cooling) the molding material in the manner of applying a heating medium having a suitable temperature into a heating medium passage provided in the wall of the structure, or by utilizing the frictional heat of the molding material per se in addition thereto.
Meantime, it can be seen from the aforesaid properties of rubbers that the purpose of vulcanizing the rubber material while it is molded within the die can be attained by feeding the rubber material to the die under an elevated pressure after the temperature of the rubber material to be extruded from the extrusion molding apparatus, for instance 80.degree. C. has been further elevated to a high temperature capable of vulcanizing the rubber material in about 10 seconds, for instance 180.degree. C. However, in view of the fact that the above mentioned conventional heating methods, as described above, rely mainly on the heat transfer using the thick steel-made structure as the medium and therefore it takes a considerably long period of time before the body to be heated, namely the rubber material, thermally responds to the heater, it is totally difficult for the conventional heating methods to achieve the above mentioned sharp rise in temperature immediately before the die as well as while preventing the material from scorching.
Further, in the case of extrusion molding, wherein molded goods are generally extruded continuously at a fixed speed, it is most preferable that thermal equilibrium is established between the heater and the object to be heated, resulting in a thermal stationary state that the temperature of the material passing through one point of the heating section can always (in terms of hour) be maintained unchanged against the fixed heating amount from the heater. Still further, even when said heating amount has been changed, it is necessary that the time required for the establishment of a new heat equilibrium therebetween should be as short as possible. Despite these requirements, however, the conventional heating methods unavoidably encounter with the trouble of unbalanced material temperatures because the heat capacity of the heating section is large as mentioned above and accordingly the log of thermal response is also large, whereby it takes a long period of time before heat equilibrium is established, during which there is caused the difference in how to be heated between the material heated by first passing through the heating section and that heated afterwards.
Still more further, it is impossible to completely eliminate the risk of an excess input even when regulating the heating amount in the stationary state as aforesaid. Once an excess heating input is generated, the material to be extruded from the die outlet after it has passed through the die is scorched and the scorched material can not pass through the obtacles in the way of the material's progress, for instance, such as mandrels, spiders or contractively modified sectional passage portions provided from the need of material velocity (pressure) distribution, whereby extrusion molding becomes impossible to continue. Therefore, the apparatus as a whole must be stopped and taken to parts for cleaning.